A New Method of weighing the Milky Way has been discovered

December 4, 2018

Milky Way is 0.96 trillion times heavier than our Sun.

Scientists have for decades tried to measure the Milky Way, a task made tedious by Earth’s position within it. Being on the outskirts of the galaxy, the inability to see it completely limits the ability to gauge its mass. The galaxy’s core houses the bulk of its luminous dense clouds of gas and dust impeding accurate measurement. Factors to take into account include, the motion, growth, distortion of orbiting satellite galaxies, our galaxies evolution, and the influence of dark matter.

Researchers have experimented with Gravitational Lensing and studying Hypervelocity stars, along with other methods to measure our galaxy’s mass but the results vary greatly from a few hundred billion to two trillion solar masses. Having said that, a new study published in the Astrophysical Journal focuses on angular momenta of the Milky Way’s satellite galaxies to weigh it.

The pioneering study led by fourth-year graduate at the University of Arizona, Ekta Patel, utilizes the 3D motions of satellite galaxies and compares their angular momenta to a simulated universe. The ultimate conclusion suggests that the Milky Way is a 0.96 trillion times the mass of our Sun. The new study exploits the lack of net change between the positions and velocities of satellite galaxies unlike its predecessors. As angular momentum of a system stays constant over time, this method allows the researchers to remove uncertainties that other approaches cannot. Consequently, it offers more reliable results. Patel elaborated their technique by saying,

“Think of a figure skater doing a pirouette. As she draws in her arms, she spins faster. In other words, her velocity changes, but her angular momentum stays the same over the whole duration of her act.”

Patel’s study Presented at the 232nd American Astronomical Society in Denver, uses satellite galaxies’ data from the Hubble Space Telescope. Comparing angular momenta of nine satellite galaxies to those of a simulated universe of 20,000 galaxies just like our own helped Patel chart nine possible ranges of values for our galaxy’s mass to arrive at the 0.96-trillion-solar-mass estimate. Except for the Magellanic Clouds, that are clearly visible to the naked eye, all other satellite galaxies are extremely hard to detect even with telescopes, making it difficult to determine their existence.

A satellite galaxy’s luminosity is often used to estimate its mass. However, orbital motions do not always align with results obtained from earlier methods. To explain imbalance between detectable and invisible mass in our universe, researchers turn to the cold dark matter theory which proposes that dark matter is made of heavy, slow-moving particles that account for roughly 85% of the universe’s matter. Dark matter weakly interacts with visible matter forming small clumps, which later form larger bodies.

Dark matter plays an important role in galaxy formation, influencing how they evolved into their present-day structures, and why they tend to form clusters. Though the theory is widely accepted, substantial experimental evidence is lacking; the precise measurement of Milky Way’s mass will test this theory. Patel explained their working mechanism in the following words:

“Currently, we know of about 50 satellite galaxies, but simulations implementing the cold dark matter theory suggest that there could be [roughly] 100-200 satellite galaxies depending on the exact mass. The gap between these measurements right now is largely due to our ability to detect (or rather not detect) these very faint dwarf galaxies that orbit our galaxy. Knowing the precise mass estimate for the Milky Way can help us determine how well this theory actually applies. Our method does not necessarily provide the most accurate or precise Milky Way mass estimate to date, but the novelty is the method, which we hope will be extremely useful in upcoming years as the observational data sets and the simulations we work with continue to grow and improve. This data is being measured with space missions like the Gaia spacecraft, so I think we’re not that far from the most precise Milky Way mass estimate to date and our method provides one way to obtain that.”

Researchers used simulated analogs from Illustris-Dark, a branch of the Illustris Project that encompasses the evolution of dark matter particles for 13.8 billion simulated years. The study uses only nine of the brightest 50 known galaxies to explore the relationship between the mass of Milky Way’s halo (predominantly dark matter) and the angular momenta of satellite galaxies. Even with an error bar of 30%, the strong relationship discovered between host halo mass and angular momenta of satellite galaxies gives this method the potential to be extended to larger datasets as the precision obtained will improve with added detailed information about satellite galaxies with future missions like the Large Synoptic Survey Telescope.

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